The shedding of rat rod outer segment (ROS) discs, and the process of phagocytosis, is a circadian phenomenon characterized morphologically by the engulfment of ROS discs by the pigment epithelium. This process occurs over a period of a few hours beginning two hours after the start of the light period. The daily synthesis of new retinal photoreceptor membranes also occurs over a period of several hours, peaking at 4 p.m., three hours before the beginning of the dark period. This event has been detected by incubation of retinas with labeled glycerol for 45 minutes followed by separation of phospholipid classes by thin-layer chromatography. S-adenosyl methionine (SAM) is a methyl donor which transmethylates phosphatidyl ethanolamine to form phosphatidyl choline. This event occurs in bovine ROS membranes and may be related to alteration of membrane fluidity. Besides phospholipids, fatty acid methyl esters and prostaglandin-like neutral lipids are transmethylated by SAM, the reaction being sensitive to light and perhaps involved in photo-transductions.